As an alternative to information systems based on polling of vehicles in a fleet of vehicles (such as buses in a public transport system), AB TRYGGIT has recently developed a system where each vehicle on its own initiates communication with a central unit and, thus, notifies the system of position, speed etc. The system which is disclosed in the Publication WO01/76105 is based on radio communication, where all vehicles transmit messages on the same radio frequency. The need for frequency bands thus is extremely limited, and by frequently transmitting messages from every vehicle, collisions, if any, between messages do not constitute a problem.
In a system of this kind, where update of the information of the system is initiated from the vehicle, it is a matter of vital importance how often such communication is transmitted. If messages are transmitted too infrequently, there is a risk that the system is not sufficiently updated, and moreover the system will be more vulnerable to any message losses.
On the other hand, frequent communications from all vehicles tend to flood the system with radio messages. Although a system according to WO01/76105 is adapted to handle such information excess, it may still put an unnecessary load on the system.